arm: mx6: cm-fx6: make it possible to not init display

Implement a cm-fx6 specific board_video_skip() to provide the option to not
initialize the display.

The new function does not init display if the environment variable "panel" is
not defined, or if it is set to an unsupported value.

Collateral changes:
- Don't use the global displays array (it's CONFIG_IMX_VIDEO_SKIP specific).
- Don't use detect_hdmi(), since env controlled init makes it unnecessary.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>

include/configs/cm_fx6.h

@@ -258,7 +258,6 @@
 #define CONFIG_VIDEO_IPUV3
 #define CONFIG_IPUV3_CLK          260000000
 #define CONFIG_IMX_HDMI
-#define CONFIG_IMX_VIDEO_SKIP
 #define CONFIG_CFB_CONSOLE
 #define CONFIG_VGA_AS_SINGLE_DEVICE
 #define CONFIG_SYS_CONSOLE_IS_IN_ENV